A conceptual framework for astrocyte function.

The participation of astrocytes in brain computation was hypothesized in 1992, coinciding with the discovery that these cells display a form of intracellular Ca2+ signaling sensitive to neuroactive molecules. This finding fostered conceptual leaps crystalized around the idea that astrocytes, once thought to be passive, participate actively in brain signaling and outputs. A multitude of disparate roles of astrocytes has since emerged, but their meaningful integration has been muddied by the lack of consensus and models of how we conceive the functional position of these cells in brain circuitry. In this Perspective, we propose an intuitive, data-driven and transferable conceptual framework we coin 'contextual guidance'. It describes astrocytes as 'contextual gates' that shape neural circuitry in an adaptive, state-dependent fashion. This paradigm provides fresh perspectives on principles of astrocyte signaling and its relevance to brain function, which could spur new experimental avenues, including in computational space.

A genetically encoded fluorescent biosensor for extracellular L-lactate.

L-Lactate, traditionally considered a metabolic waste product, is increasingly recognized as an important intercellular energy currency in mammals. To enable investigations of the emerging roles of intercellular shuttling of L-lactate, we now report an intensiometric green fluorescent genetically encoded biosensor for extracellular L-lactate. This biosensor, designated eLACCO1.1, enables cellular resolution imaging of extracellular L-lactate in cultured mammalian cells and brain tissue.

Obesity-induced astrocyte dysfunction impairs heterosynaptic plasticity in the orbitofrontal cortex.

Overconsumption of highly palatable, energy-dense food is considered a key driver of the obesity pandemic. The orbitofrontal cortex (OFC) is critical for reward valuation of gustatory signals, yet how the OFC adapts to obesogenic diets is poorly understood. Here, we show that extended access to a cafeteria diet impairs astrocyte glutamate clearance, which leads to a heterosynaptic depression of GABA transmission onto pyramidal neurons of the OFC. This decrease in GABA tone is due to an increase in extrasynaptic glutamate, which acts via metabotropic glutamate receptors to liberate endocannabinoids. This impairs the induction of endocannabinoid-mediated long-term plasticity. The nutritional supplement, N-acetylcysteine rescues this cascade of synaptic impairments by restoring astrocytic glutamate transport. Together, our findings indicate that obesity targets astrocytes to disrupt the delicate balance between excitatory and inhibitory transmission in the OFC.

Whole brain irradiation in mice causes long-term impairment in astrocytic calcium signaling but preserves astrocyte-astrocyte coupling.

Whole brain irradiation (WBI) therapy is an important treatment for brain metastases and potential microscopic malignancies. WBI promotes progressive cognitive dysfunction in over half of surviving patients, yet, the underlying mechanisms remain obscure. Astrocytes play critical roles in the regulation of neuronal activity, brain metabolism, and cerebral blood flow, and while neurons are considered radioresistant, astrocytes are sensitive to γ-irradiation. Hallmarks of astrocyte function are the ability to generate stimulus-induced intercellular Ca2+ signals and to move metabolic substrates through the connected astrocyte network. We tested the hypothesis that WBI-induced cognitive impairment associates with persistent impairment of astrocytic Ca2+ signaling and/or gap junctional coupling. Mice were subjected to a clinically relevant protocol of fractionated WBI, and 12 to 15 months after irradiation, we confirmed persistent cognitive impairment compared to controls. To test the integrity of astrocyte-to-astrocyte gap junctional coupling postWBI, astrocytes were loaded with Alexa-488-hydrazide by patch-based dye infusion, and the increase of fluorescence signal in neighboring astrocyte cell bodies was assessed with 2-photon microscopy in acute slices of the sensory-motor cortex. We found that WBI did not affect astrocyte-to-astrocyte gap junctional coupling. Astrocytic Ca2+ responses induced by bath administration of phenylephrine (detected with Rhod-2/AM) were also unaltered by WBI. However, an electrical stimulation protocol used in long-term potentiation (theta burst), revealed attenuated astrocyte Ca2+ responses in the astrocyte arbor and soma in WBI. Our data show that WBI causes a long-lasting decrement in synaptic-evoked astrocyte Ca2+ signals 12-15 months postirradiation, which may be an important contributor to cognitive decline seen after WBI.

Author Correction: Stress gates an astrocytic energy reservoir to impair synaptic plasticity.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Stress gates an astrocytic energy reservoir to impair synaptic plasticity.

Astrocytes support the energy demands of synaptic transmission and plasticity. Enduring changes in synaptic efficacy are highly sensitive to stress, yet whether changes to astrocyte bioenergetic control of synapses contributes to stress-impaired plasticity is unclear. Here we show in mice that stress constrains the shuttling of glucose and lactate through astrocyte networks, creating a barrier for neuronal access to an astrocytic energy reservoir in the hippocampus and neocortex, compromising long-term potentiation. Impairing astrocytic delivery of energy substrates by reducing astrocyte gap junction coupling with dominant negative connexin 43 or by disrupting lactate efflux was sufficient to mimic the effects of stress on long-term potentiation. Furthermore, direct restoration of the astrocyte lactate supply alone rescued stress-impaired synaptic plasticity, which was blocked by inhibiting neural lactate uptake. This gating of synaptic plasticity in stress by astrocytic metabolic networks indicates a broader role of astrocyte bioenergetics in determining how experience-dependent information is controlled.

Stress-induced structural and functional modifications of astrocytes-Further implicating glia in the central response to stress.

An organism's response to stress requires activation of multiple brain regions. This can have long-lasting effects on synaptic transmission and plasticity that likely provide adaptive benefits. Recent evidence implicates not only neurones, but also glial cells in the regulation of the central response to stress. Intense, repeated or uncontrolled stress has been implicated in the emergence of multiple neuropsychiatric conditions. Human studies have consistently observed glial dysfunction in mood and stress disorders such as major depression. Interestingly animal models of stress have recapitulated glial abnormalities that are comparable to the human condition, validating the use of rodent models for the study of stress disorders. In this review we will focus upon one family of glia, the astrocytes, and describe the evidence to date that links astrocytes to possible stress-related disorders.

Cholecystokinin Switches the Plasticity of GABA Synapses in the Dorsomedial Hypothalamus via Astrocytic ATP Release.

Whether synapses in appetite-regulatory brain regions undergo long-term changes in strength in response to satiety peptides is poorly understood. Here we show that following bursts of afferent activity, the neuromodulator and satiety peptide cholecystokinin (CCK) shifts the plasticity of GABA synapses in the dorsomedial nucleus of the hypothalamus of male Sprague Dawley rats from long-term depression to long-term potentiation (LTP). This LTP requires the activation of both type 2 CCK receptors and group 5 metabotropic glutamate receptors, resulting in a rise in astrocytic intracellular calcium and subsequent ATP release. ATP then acts on presynaptic P2X receptors to trigger a prolonged increase in GABA release. Our observations demonstrate a novel form of CCK-mediated plasticity that requires astrocytic ATP release, and could serve as a mechanism for appetite regulation.SIGNIFICANCE STATEMENT Satiety peptides, like cholecystokinin, play an important role in the central regulation of appetite, but their effect on synaptic plasticity is not well understood. The current data provide novel evidence that cholecystokinin shifts the plasticity from long-term depression to long-term potentiation at GABA synapses in the rat dorsomedial nucleus of the hypothalamus. We also demonstrate that this plasticity requires the concerted action of cholecystokinin and glutamate on astrocytes, triggering the release of the gliotransmitter ATP, which subsequently increases GABA release from neighboring inhibitory terminals. This research reveals a novel neuropeptide-induced switch in the direction of synaptic plasticity that requires astrocytes, and could represent a new mechanism by which cholecystokinin regulates appetite.

Dynamics of surface neurotransmitter receptors and transporters in glial cells: Single molecule insights.

The surface dynamics of neurotransmitter receptors and transporters, as well as ion channels, has been well-documented in neurons, revealing complex molecular behaviour and key physiological functions. However, our understanding of the membrane trafficking and dynamics of the signalling molecules located at the plasma membrane of glial cells is still in its infancy. Yet, recent breakthroughs in the field of glial cells have been obtained using combination of superresolution microscopy, single molecule imaging, and electrophysiological recordings. Here, we review our current knowledge on the surface dynamics of neurotransmitter receptors, transporters and ion channels, in glial cells. It has emerged that the brain cell network activity, synaptic activity, and calcium signalling, regulate the surface distribution and dynamics of these molecules. Remarkably, the dynamics of a given neurotransmitter receptor/transporter at the plasma membrane of a glial cell or neuron is unique, revealing the existence of cell-type specific regulatory pathways. Thus, investigating the dynamics of signalling proteins at the surface of glial cells will likely shed new light on our understanding of glial cell physiology and pathology.

Steady-State Free Ca2+ in Astrocytes Is Decreased by Experience and Impacts Arteriole Tone.

Astrocytes can control basal synaptic strength and arteriole tone via their resting Ca2+ activity. However, whether resting astrocyte Ca2+ can adjust to a new steady-state level, with an impact on surrounding brain cells, remains unknown. Using two-photon Ca2+ imaging in male rat acute brain slices of the somatosensory neocortex, we found that theta burst neural activity produced an unexpected long-lasting reduction in astrocyte free Ca2+ in the soma and endfeet. The drop in intracellular Ca2+ was attenuated by antagonists targeting multiple ionotropic and metabotropic glutamate receptors, and intracellular cascades involved Ca2+ stores and nitric oxide. The reduction in astrocyte endfoot Ca2+ was coincident with an increase in arteriole tone, and both the Ca2+ drop and the tone change were prevented by an NMDA receptor antagonist. Astrocyte patch-clamp experiments verified that the glutamate receptors in question were located on astrocytes and that Ca2+ changes within astrocytes were responsible for the long-lasting change in arteriole diameter caused by theta burst neural activity. In astrocytes from animals that lived in an enriched environment, we measured a relatively lower resting Ca2+ level that occluded any further drop in Ca2+ in response to theta burst activity. These data suggest that electrically evoked patterns of neural activity or natural experience can adjust steady-state resting astrocyte Ca2+ and that the effect has an impact on basal arteriole diameter.SIGNIFICANCE STATEMENT The field of astrocyte-neuron and astrocyte-arteriole interactions is currently in a state of refinement. Experimental evidence ex vivo suggests that direct manipulation of astrocyte-free Ca2+ regulates synaptic signaling and local blood flow control; however, in vivo experiments fail to link synaptically evoked astrocyte Ca2+ transients and immediate changes to various astrocyte-mediated processes. To clarify this discrepancy, we examined a different aspect of astrocyte Ca2+: the resting, steady-state free Ca2+ of astrocytes, its modulation, and its potential role in the tonic regulation of surrounding brain cells. We found that ex vivo or in vivo neural activity induced a long-lasting reduction in resting free astrocyte Ca2+ and that this phenomenon changed arteriole tone.

Astroglial glutamate transporters in the brain: Regulating neurotransmitter homeostasis and synaptic transmission.

Astrocytes, the major glial cell type in the central nervous system (CNS), are critical for brain function and have been implicated in various disorders of the central nervous system. These cells are involved in a wide range of cerebral processes including brain metabolism, control of central blood flow, ionic homeostasis, fine-tuning synaptic transmission, and neurotransmitter clearance. Such varied roles can be efficiently carried out due to the intimate interactions astrocytes maintain with neurons, the vasculature, as well as with other glial cells. Arguably, one of the most important functions of astrocytes in the brain is their control of neurotransmitter clearance. This is particularly true for glutamate whose timecourse in the synaptic cleft needs to be controlled tightly under physiological conditions to maintain point-to-point excitatory transmission, thereby limiting spillover and activation of more receptors. Most importantly, accumulation of glutamate in the extracellular space can trigger excessive activation of glutamatergic receptors and lead to excitotoxicity, a trademark of many neurodegenerative diseases. It is thus of utmost importance for both physiological and pathophysiological reasons to understand the processes that control glutamate time course within the synaptic cleft and regulate its concentrations in the extracellular space. © 2017 Wiley Periodicals, Inc.

Surface diffusion of astrocytic glutamate transporters shapes synaptic transmission.

Control of the glutamate time course in the synapse is crucial for excitatory transmission. This process is mainly ensured by astrocytic transporters, high expression of which is essential to compensate for their slow transport cycle. Although molecular mechanisms regulating transporter intracellular trafficking have been identified, the relationship between surface transporter dynamics and synaptic function remains unexplored. We found that GLT-1 transporters were highly mobile on rat astrocytes. Surface diffusion of GLT-1 was sensitive to neuronal and glial activities and was strongly reduced in the vicinity of glutamatergic synapses, favoring transporter retention. Notably, glutamate uncaging at synaptic sites increased GLT-1 diffusion, displacing transporters away from this compartment. Functionally, impairing GLT-1 membrane diffusion through cross-linking in vitro and in vivo slowed the kinetics of excitatory postsynaptic currents, indicative of a prolonged time course of synaptic glutamate. These data provide, to the best of our knowledge, the first evidence for a physiological role of GLT-1 surface diffusion in shaping synaptic transmission.

Interleukin-1alpha and HMGB1 mediate hippocampal dysfunction in SIGIRR-deficient mice.

Single-Ig-interleukin-1 related receptor (SIGIRR) is a member of the interleukin (IL)-1/Toll-like receptor (TLR) family. It negatively regulates inflammation, rendering SIGIRR(-/-) mice more susceptible to inflammatory challenge. This susceptibility extends to the brain, where increased responsiveness to lipopolysaccharide has been observed in SIGIRR-deficient mice. While this is likely due to enhanced TLR4-mediated signaling, the functional consequences of these changes have not yet been described. In the current study, we have investigated the impact of SIGIRR deficiency on hippocampal function, and show that novel object recognition, spatial reference memory, and long-term potentiation (LTP) were impaired in SIGIRR(-/-) mice. These changes were accompanied by increased expression of IL-1RI and TLR4, and upregulation of their downstream signaling events, namely IRAK1 (IL-1R-associated kinase 1), c-Jun N-terminal protein kinase (JNK), and nuclear factor κB (NF-κB). The deficit in LTP was attenuated by the endogenous IL-1 receptor antagonist (IL-1ra) and an anti-TLR4 antibody, and also by inhibition of JNK and NF-κB. We propose that IL-1RI is activated by IL-1α and TLR4 is activated by the endogenous agonist, high mobility group box 1 (HMGB1), as we identified enhanced expression of both cytokines in the hippocampus of SIGIRR(-/-) mice. Additionally, application of HMGB1 increased the activation of JNK and NF-κB and was found to be detrimental to LTP in a TLR4-dependent manner. These findings highlight the functional role of SIGIRR in regulating inflammatory-mediated synaptic and cognitive decline, and describe evidence of the key role of HMGB1 in this process.